As methods for obtaining colored hard copies, there have so far been studied color recording techniques such as ink jet recording, electrophotography and thermal transfer recording. Of these methods, thermal transfer recording has, not necessarily limited to the case of color recording, advantages that it is easy to handle and maintain, allows downsizing of apparatus and requires less cost.
This thermal transfer recording includes the heat melt transfer method and the thermo-diffusion transfer method; the latter is also called the thermo-sublimation transfer method, because it uses a sublimation-type thermal transfer recording ink sheet having on a support an ink layer containing a sublimation dye which sublimates on heating and transfers, through diffusion, onto a thermal transfer recording image receiving sheet.
This thermo-diffusion transfer method can control the gradation of an image by varying the amount of dye to be transferred correspondingly to the change in the amount of heat energy applied by a thermal head. Accordingly, it has a distinctive advantage that it can easily form multicolor printed images having a continuous change in color shades by performing a superposition-recording using the three primary colors of cyan, magenta and yellow.
With such an advantage, the thermal transfer recording method of thermo-diffusion transfer type has come to be extensively used in making image recording bodies to record gradational color images such as color photographic images.
However, conventional thermal transfer recording image receiving sheets used in this thermal transfer recording method have problems of causing uneven printing owing to surface irregularities or adhesion of foreign matter and yielding a low transfer sensitivity and an insufficient transfer density.
As a measure to solve these problems, use of a support of porous structure is proposed as described in Japanese Pat. O.P.I. Pub. Nos. 112693/1986, 148293/1987, 152793/1987, 1595/1988, 231984/1988, 44781/1989, 47536/1989, 168493/1989, 92592/1990, etc.
Further, methods for laminating a porous intermediate layer between the support and the image receiving layer are proposed as seen in Japanese Pat. O.P.I. Pub. Nos. 270192/1986, 87286/1988, 122593/1988, 145192/1989, 280586/1989, 89690/1990, 241740/1990, 76687/1991, etc. and methods for laminating an intermediate layer having a cushioning function between the support and the image receiving sheet are proposed as seen in Japanese Pat. O.P.I. Pub. Nos. 144394/1986, 258793/1986, 146693/1987, 151393/1987, 5885/1989, 26497/1989, etc.
In these methods, however, the unevenness in images, though fairly reduced, cannot be prevented enough at all times; moreover, a sufficient transfer density and a sufficient transfer sensitivity cannot be obtained in times; in the case of intermediate layers having a porous structure, curling cannot be prevented enough on occasion for their insufficient heat stability and dimensional stability. Further, when an intermediate layer is formed by a coating method, curling tends to occur in the drying process after coating because of the thermal shrinkage of the coated layer, besides an environmental problem which may be caused by evaporation of solvents.
In order to prevent such disadvantages, there is proposed extrusion lamination of a polyolefine resin on a base made of wood pulp or natural pulp as described in Japanese Pat. O.P.I. Pub. Nos. 263081/1989, 106397/1990, 305688/1990 and U.S. Pat. Nos. 4,774,224, 4,999,335. When paper is used as a base, curling can be prevented, but there occur at times uneven images attributable to the surface waviness of fibers in the paper as well as insufficient transfer sensitivities and transfer densities due to diffusion of heat during recording.